Inch by inch, block by block, floods consumed Australia's third-largest city, creeping across suburban gardens today and streaming through downtown streets darkened by power outages and largely emptied of people.

The waters poured into Brisbane, knocking down traffic lights on some streets, after marching across Queensland for weeks. Roads shut throughout the city, and people moved about in kayaks, rowboats and even on surfboards. Boats torn from their moorings floated down an engorged river.

Residents of the city's low-lying areas headed in the thousands for higher ground, while others chose to ride it out as the waters approached their expected peak early tomorrow morning.

Some neighbours in East Brisbane fired up a barbecue, put some beer on ice and starting grilling sausages while watching the water spill from a picturesque creek and creep up the gentle slope toward their homes.

"We're having a bit of a barbecue because we've got no electricity," said Bob Vilgan, an 80-something dressed like most of the neighbours in shorts, a T-shirt and sandals. "The idea is to get everyone fed who is in with our crowd, have a few tiny tipples and get back home."

The flooding, which has killed 22 people since late November, has submerged dozens of towns — some three times — and left an area the size of Germany and France combined under water. Highways and rail lines have been washed away in the disaster, which is shaping up to be Australia's costliest ever.

With at least 43 people missing, the death toll is expected to rise. Many of those unaccounted for disappeared from around Toowoomba, a city west of Brisbane that saw massive flash floods sweep away cars, road signs and people. Twelve died in that flood alone.

The toll has shocked Australians, no strangers to deadly natural disasters like the wildfires that killed 173 in a single day two years ago.

One tale has particularly transfixed the country: a 13-year-old boy caught in the flood who told strangers to save his 10-year-old brother first and died as a result.

Jordan and Blake Rice were in the car with their mother, Donna, when a wall of water pummeled Toowoomba on Monday. After the torrent of water knocked one rescuer over, another man managed to reach the car, The Australian newspaper reported. At Jordan's insistence, he pulled Blake out first, according to a third brother, Kyle.

"Courage kicked in, and he would rather his little brother would live," the 16-year-old told the newspaper. Jordan and his mother were washed away before the men were able to get back to them. By today, Jordan's name was among the top 10 most used terms on Twitter, as a wave of tweets hailed him as a "true hero" of the Queensland floods.

In contrast to the wall of water that swallowed Toowoomba, Brisbane's crisis has been marked by the waters' slow but steady progress.

"I was quite panicked after seeing Toowoomba," said Ali Cook, one of the neighbours at the East Brisbane barbecue. "But it's been such a slow rise. It's still rising quite a lot."

Emergency sirens blared across the city as the floodwaters entered an empty downtown and began swamping neighbourhoods.

The surging, muddy waters reached the tops of traffic lights in some parts, and Mayor Campbell Newman said at least 20,000 homes would likely be damaged. Brisbane's office buildings stood empty with the normally bustling central business district transformed into a watery ghost town.

Police went door-to-door in some neighbourhoods advising people to leave. Five evacuation centres were open with room for 16,000 people.

The Brisbane River is expected to reach its height tomorrow, at a depth slightly lower the that of 1974 floods that swept the city. Bligh said the news was welcome, but of little comfort.

"This is still a major event, the city is much bigger, much more populated and has many parts under flood that didn't even exist in 1974," she said. "We are still looking at an event which will cripple parts of our city."

The waters have overwhelmed a dam built to protect the city after the 1974 deluge. Officials have opened the floodgates of the dam to prevent a greater disaster, contributing to the flooding downtown.

Though the full extent of the damage won't be known until the water is gone, even before Brisbane was threatened, Queensland Premier Anna Bligh estimated a cleanup and rebuilding to total around $5bn.

Add to that, the damage to economy: Queensland's coal industry has virtually shut down, costing millions in deferred exports and sending global prices higher. Vegetables, fruit and sugarcane crops in the rich agricultural region have been wiped out, and prices are due to skyrocket as a result.

Water levels were expected to stay at peak levels until at least Saturday, but many people won't be able to access their homes for several days beyond that, Bligh said.

Energex, Brisbane's main power company, started switching off electricity to some parts of the city as a precaution against electrocution. Almost 70,000 homes were without power across Queensland by this afternoon, Bligh said.

Residents who had spent two days preparing took cover on higher ground while others scrambled to move their prized possessions to the top floors of their homes. Some stacked furniture on their roofs. Supermarkets, which had experienced a run on bottled water, food and other goods in the previous days, stayed closed today

Darren Marchant spent all day moving furniture and other household goods to the top floor of his home, near the river in the low-lying Brisbane suburb of Yeronga. He and two neighbours watched in awe as dozens of expensive boats and pontoons drifted past.

"We were watching all kinds of debris floating down the river — one of the (neighbour's) pontoons just floated off," he said. "It was amazing."

West of Brisbane, the city of Ipswich, home to about 15,000 people, 3,000 properties were swamped by the waters heading Brisbane's way, and 1,100 people had fled to evacuation centres, Mayor Paul Pisasale said. The floods also reached further into New South Wales, causing about 3,000 people to leave their homes there.

In Ipswich, video showed horses swimming through the brown waters, pausing to rest their heads on the roof of a house — the only dry spot they could reach.